The present invention relates generally to regenerator-based oscillating-gas engines and refrigerators, and, more particularly, to thermoacoustic engines and refrigerators, including Stirling engines and refrigerators and pulse-tube refrigerators, and hybrids thereof.
According to thermodynamic principles, acoustic power in a gas (a nonzero time average product of oscillating pressure and oscillating volumetric flow rate) is as valuable as other forms of work, such as electrical power, rotating shaft power, hydraulic power, and the like. For example, acoustic power can be used to produce refrigeration, such as in orifice pulse-tube refrigerators; it can be used to produce electricity, via linear alternators; and it can be used to generate rotating shaft power with a Wells turbine. Furthermore, acoustic power can be created from heat in a variety of heat engines such as Stirling engines and thermoacoustic engines. See, for example Thermoacoustics: a unifying perspective for some engines and refrigerators, G. W. Swift, to be published by the Acoustical Society of America in 2002; available in pre-publication format at http://www.lanl.gov/projects/thermoacoustics/Book/index.html.
Historically, Stirling""s hot-air engine of the early 19th century was the first regenerator-based heat engine to use oscillating pressure and oscillating volumetric flow rate in a gas in a sealed system, although the time-averaged product of oscillating pressure and oscillating volumetric flow rate was not called acoustic power. Since then, a variety of related engines and refrigerators have been developed, including Stirling refrigerators, Ericsson engines, orifice pulse-tube refrigerators, standing-wave thermoacoustic engines and refrigerators, free-piston Stirling engines and refrigerators, and thermoacoustic-Stirling hybrid engines and refrigerators. Combinations thereof, such as the Vuilleumier refrigerator and the thermoacousically driven orifice pulse-tube refrigerator, have provided heat-driven refrigeration.
Much of the evolution of this entire family of acoustic-power thermodynamic technologies has been driven by the search for higher efficiency, greater reliability, and lower fabrication cost.
FIG. 1 shows one example of such a prior art regenerator-based engine: a thermoacoustic-Stirling hybrid engine, described in xe2x80x9cTraveling Wave Device With Mass Flux Suppression,xe2x80x9d G. W. Swift, U.S. Pat. No. 6,032,464, Mar. 7, 2000; xe2x80x9cA thermoacoustic-Stirling heat engine,xe2x80x9d S. Backhaus et al., Nature 399, 335-338 (1999); xe2x80x9cA thermoacoustic-Stirling heat engine: Detailed study,xe2x80x9d S. Backhaus et al., J. Acoust. Soc. Am. 107, 3148-3166 (2000). The engine delivers acoustic power 10 to an unspecified load (e.g., a linear alternator or any of the aforementioned refrigerators) to its right. High-temperature heat, such as from a flame or from nuclear fuel, is added to the engine at hot heat exchanger 12, most of the ambient-temperature waste heat is removed from the engine at ambient heat exchanger 14, and oscillations of the gas are thereby caused.
As in all of the regenerator-based engines listed above, the conversion of heat to acoustic power occurs in regenerator 16, which is a solid matrix smoothly spanning the temperature difference between hot heat exchanger 12 and ambient heat exchanger 14, and containing small pores through which the gas oscillates. The pores must be small enough that the gas in them is in excellent local thermal contact with the heat capacity of the solid matrix. Proper design of toroidal acoustic network 18 (including, principally, inertance 22 and compliance 24) causes the gas in the pores of regenerator 16 to move toward hot heat exchanger 12 while the pressure is high and toward ambient heat exchanger 14 while the pressure is low. The oscillating thermal expansion and contraction of the gas in regenerator 16, attending its oscillating motion along the temperature gradient in the pores, is therefore temporally phased with respect to the oscillating pressure so that the thermal expansion occurs while the pressure is high and the thermal contraction occurs while the pressure is low.
Those skilled in the art understand that another way to view the operation of the thermoacoustic-Stirling hybrid engine, and indeed all regenerator-based engines including all Stirling and traveling-wave engines, is that acoustic power {dot over (E)}0 flows into the ambient end (i.e., the end adjacent to ambient heat exchanger 14) of regenerator 16, is amplified in regenerator 16 by the temperature gradient in regenerator 16, and flows out of the hot end (i.e., the end adjacent to hot heat exchanger 12) of regenerator 16. Ideally, the acoustic-power amplification factor in regenerator 16 is equal to the ratio of hot temperature to ambient temperature, both temperatures being measured in absolute units such as Kelvin. In FIG. 1, the acoustic power {dot over (E)}H flowing out of the hot end of regenerator 16 through thermal buffer tube 72 splits into two portions {dot over (E)}load and {dot over (E)}fb at the resonator junction, with the required amount {dot over (E)}0 flowing into the ambient end of regenerator 16 to provide the original acoustic power for amplification, and the rest {dot over (E)}load being delivered to the load. Hence, circulating acoustic power flows through regenerator 16 from ambient to hot and is amplified therein.
Two heat exchangers 12,14, one adjacent to each end of regenerator 16, are vital for the operation of such an engine. Both of these heat exchangers typically put the oscillating internal gas in intimate thermal contact with a steadily flowing external fluid such as water, air, or combustion products. Here, hot heat exchanger 12 must supply heat to the internal, thermodynamic working gas from an external heat source such as combustion products flowing from a burner. Similarly, ambient heat exchanger 14 must remove heat from the internal gas, rejecting that heat to an external heat sink such as a flowing stream of ambient-temperature water or air.
In common practice in heat exchanger design, both the internal gas and the external fluid are subdivided into many parallel portions that are interwoven, most often in cross flow. In a cross flow shell and tube heat exchanger, the internal gas often oscillates axially through a large number of parallel tubes, while the external fluid flows around the outside of the tubes perpendicular to the tube axes. In a finned tube heat exchanger, the external fluid may flow axially through a number of parallel tubes, while the internal gas oscillates around the finned outsides of the tubes perpendicular to the tube axes.
A similar description can be provided for a prior art thermoacoustic-Stirling hybrid refrigerator described in xe2x80x9cTraveling Wave Device With Mass Flux Suppression,xe2x80x9d G. W. Swift, et al., U.S. Pat. No. 6,032,464, Mar. 7, 2000; xe2x80x9cAcoustic recovery of lost power in pulse-tube refrigerators,xe2x80x9d G. W. Swift et al., J. Acoust. Soc. Am. 105, 711-724 (1999), shown in FIG. 2. Similar to the thermoacoustic-Stirling hybrid engine shown in FIG. 1, essential features of the refrigerator are that acoustic power {dot over (E)}0 must flow through regenerator 32 from ambient heat exchanger 42 to cold heat exchanger, 38, acoustic power is thereby attenuated, and the pores of regenerator 32 must be small enough to provide excellent thermal contact between the gas and the solid matrix. Proper design of toroidal acoustic network 34 (including, principally, inertance 36 and compliance 38) causes the gas in the pores of regenerator 32 to move toward cold heat exchanger 38 while the pressure is high and toward ambient heat exchanger 42 while the pressure is low. Acoustic power {dot over (E)}C moves through thermal buffer tube 82 to combine with the input power {dot over (E)}drive to return through toroidal acoustic network 34. The oscillating entropy of the gas in regenerator 32, attending its oscillating pressure, is therefore temporally phased with respect to the oscillatory motion along the temperature gradient in the pores so that heat is pumped through regenerator 32 from cold heat exchanger 38 toward ambient heat exchanger 42.
As described for the engine above, two heat exchangers 38, 42, one adjacent to each end of regenerator 32, are required for the operation of such a refrigerator. Cold heat exchanger 38 must remove heat from the external heat load, such as flowing indoor air to be cooled, transferring that heat into the internal gas. Similarly, ambient heat exchanger 42 must remove heat from the internal gas, rejecting that heat to an external heat sink, such as a flowing stream of ambient-temperature water or air. Both heat exchangers 38, 42 typically put the oscillating internal gas in intimate thermal contact with a steadily flowing external fluid such as water or air, with both the internal gas and the external fluid subdivided into many parallel portions that are interwoven, most often in cross flow.
Another well-known form of regenerator-based refrigerator is the orifice pulse-tube refrigerator, described in xe2x80x9cA review of pulse-tube refrigeration,xe2x80x9d R. Radebaugh, Adv. Cryogenic Eng. 35, 1191-1205 (1990), illustrated in FIG. 3. The oscillating motion and pressure, and resulting thermodynamic phenomena, in regenerator 52 and adjacent heat exchangers 54,56 are the same as those in the thermoacoustic-Stirling hybrid refrigerator shown in FIG. 2. Input acoustic energy {dot over (E)}0 is amplified and output as {dot over (E)}C into pulse tube 104. However, whereas the thermoacoustic-Stirling hybrid refrigerator shown in FIG. 2 uses a toroidal acoustic network, the orifice pulse-tube refrigerator accomplishes the same thermodynamic phenomena with a simpler acoustic network 58 having no torus.
Those skilled in the art will understand that xe2x80x9cambientxe2x80x9d temperature in this discussion refers to the temperature at which waste heat is rejected, and need not always be a temperature near ordinary room temperature. For example, a cryogenic refrigerator intended to liquefy hydrogen at 20 Kelvin might reject its waste heat to a liquid-nitrogen stream at 77 Kelvin; for the purposes of this cryogenic refrigerator, xe2x80x9cambientxe2x80x9d would be 77 Kelvin. Those skilled in the art will also understand that xe2x80x9crefrigeratorxe2x80x9d includes heat pumps.
Those skilled in the art also understand that, in the design of the gas dynamics in the regenerators of such engines and refrigerators, there is an optimal choice for the ratio of oscillating pressure to oscillating volumetric flow rate, while keeping their product at a desired value in order to keep the acoustic power at a desired level. Viscous dissipation of acoustic power in the regenerator is undesirable, and is avoided by keeping the oscillating volumetric flow rate as low as possible. On the other hand, the need to achieve significant heat transfer pushes designs toward high volumetric flow rate, and the need to avoid pressure-hysteresis losses limits the oscillating pressure. Hence, the ratio of oscillating pressure to oscillating volumetric flow rate should be high, but not too high. Typical design optimizations balancing these phenomena yield a ratio of oscillating pressure to oscillating volumetric flow rate on the order of magnitude of 10 times the gas density times the gas sound speed divided by the cross-sectional area of the regenerator.
Unfortunately, the aforementioned constraint on volumetric flow rate is equivalent to a bound on volumetric displacement, which in turn limits both the gas volume that each heat exchanger encloses and the gas volume that can be allocated to the space between the regenerator and a heat exchanger (e.g., in order to accommodate changes in cross section or direction between the regenerator and the heat exchanger). Hence, the heat exchangers that are adjacent to the regenerator in regenerator-based engines and refrigerators are typically short in the direction of the oscillatory motion of the gas, as broad in cross-sectional area as the regenerator itself, and abutted closely to the regenerator, as shown in FIG. 1 for hot heat exchanger 12 and ambient heat exchanger 14 and in FIGS. 2 and 3 for cold heat exchangers 38, 54 and ambient heat exchangers 42, 56.
These geometrical constraints make it difficult to build such heat exchangers cheaply and they make it difficult to achieve excellent heat transfer in such heat exchangers. The short, broad aspect ratio of such heat exchangers usually causes them to be made of many parallel subunits, so that the number of parts that must be handled, assembled, and bonded in a leak-tight fashion is large, causing high fabrication costs. The volume constraint leads either to low surface area or to a multiplicity of tiny passages, causing either poor heat transfer or high cost. The broad aspect ratio can also lead to low velocities, so that heat-transfer coefficients are low. The geometrical constraints can sometimes also preclude good heat transfer or low pressure drop on the non-thermoacoustic side of the heat exchanger, such as in the combustion-products stream of a burner-heated hot heat exchanger. The geometrical constraints can also lead to structural engineering challenges. For example, in an engine with a red-hot heat exchanger, such constraints make it difficult to provide the slight structural flexibility needed to accommodate slightly different thermal expansions in different portions of the heat exchanger, which can arise from slightly different hot temperatures in different portions of the heat exchanger.
Accordingly, in regenerator-based engines and refrigerators, it is desirable to provide greater geometrical freedom for one or more heat exchangers, in order that the heat exchanger(s) can have greater surface area, higher heat-transfer coefficient, and more structural design options. It is further desirable to make the oscillating volumetric flow rate and the oscillating volumetric displacement through a heat exchanger greater than that through the adjacent regenerator, in order that the heat exchanger can have greater surface area, higher flow velocity, and more structural design options.
Various features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention includes a regenerator-based engine or refrigerator having a regenerator with two ends at two different temperatures, through which a gas oscillates at a first oscillating volumetric flow rate in the direction between the two ends and in which the pressure of the gas oscillates, and first and second heat exchangers, each of which is at one of the two different temperatures. A dead-end side branch into which the gas oscillates has compliance and is connected adjacent to one of the ends of the regenerator to form a second oscillating gas flow rate additive with the first oscillating volumetric flow rate, the compliance having a volume effective to provide a selected total oscillating gas volumetric flow rate through the first heat exchanger.